Need Advice: I won't take FT offer because I am delaying graduation

Last year I got an offer at a bank (not in IB) and I realized I could delay my graduation by a year to have a chance at true IB recruiting (I didn't partake in it earlier that year). I got a great offer for next summer before I walked in the door this summer. I'm nearing the end of my internship and I want to know how I should decline the FT offer they will likely give me.

I feel ashamed, as I should. I misled these people who have been nice to me into thinking they could turn me into a full time analyst. I used them to get something better, and that's low. I had spoken with the head of career services at my school (before starting this) and he said "I don't think you have an obligation to tell them" and seemed okay with what I'm doing, but I still feel a bit ashamed. But it's too late now and I can't get out of what I have next summer, and I don't want to. I'm really looking forward to IB next summer.

I know people at different banks talk to each other. I don't want that to happen here. The people next summer think I'm a sophomore currently and I don't want that to change. 

I need some advice on how to not accept the FT offer this summer without raising red flags or anything. Any advice is appreciated.